fix(pi): expose native skills through $ instead of /

[swear01]

Summary

• split Pi get_commands inventory by native source: skills go to HAPI $, prompt/extension commands stay in /
• normalize Pi's skill:<name> transport prefix to $<name> and translate HAPI invocations back to /skill:<name>
• keep Pi skill commands first when attachments are present, so Pi performs native skill expansion
• stop Pi and Kimi from falling back to Claude built-in slash commands

Pi RPC documents source: "skill" entries as /skill:<name> commands and expands them for prompt/steer input. Gemini legacy is intentionally out of scope.

Fixes #1285
Parent: #1284

Verification

• bun typecheck — exit 0
• bun run test — 4,243 passed; 3 skipped; 0 failed
• focused Pi/web tests — 17 passed

[github-actions]

Findings

• No issues found in the added or modified lines.

Questions

• None.

Summary

• Review mode: initial. No Blocker, Major, Minor, or Nit findings. Residual risk: the Pi namespace and prompt rewrite are covered by helper tests, but not by an end-to-end test against a real Pi RPC process.

Testing

• Not run (automation): Bun is unavailable in this review environment; the GitHub test check was pending at review time.

HAPI Bot